1. Technical Field of the Invention
Filters are one of the most commonly used components in communication systems. Filters shape waveforms, match impedance, inhibit harmonic emissions, reduce system and image noise, lower interference, etc. The proposed filter can be extensively used in, for example, wire and wireless communication equipment and handsets for such purposes.
2. Description of Related Art
More than five filters may be used in a modern communication system. Therefore, the performance, size, and cost of filters are very important.
Conventionally, substrates with high dielectric constant are used to reduce filter size. There are two main disadvantages with this conventional solution. First, such filters are difficult to manufacture and process because of the resultant fine line width. Secondly, the performance of the filter is quite sensitive to any layout variations and thus requires extensive post-tuning of multiple components to compensate for such variations. Overcoming the above defects greatly increases the cost of the filter.
In MLC/LTCC (Multilayer Ceramic/Low Temperature Co-fired Ceramic) applications, in order to reduce the size and cost of system module it is desirable that sub-modules be integrated together into a single module. Most passive components, including capacitors, inductors, resistors, filters, transmission lines, DC and interconnect lines, etc., are built into multilayer substrates.
The values of these built-in components must be controlled precisely because they are hard to tune. This also limits the use of substrates with high dielectric constant in these applications. In addition, the structure of filter itself must be suitable to be built into a substrate.
One such conventional multilayer bandpass filter is described in Nakai et al. (U.S. Pat. No. 5,523,729). FIGS. 1A, 1B, and 1C are the equivalent circuits from Nakai et al. and are representative of commercial multilayer bandpass filters. These circuits consist of input and output coupling capacitors, resonant capacitors, resonant and coupling inductors, and a loss-pole shifting capacitor.
The response of the circuit shown in FIG. 1A has one loss pole near the lower side of the passband. FIGS. 1B and 1C result in two loss poles: one loss pole is located at the lower and the other at the higher end of the passband.
Extensive post-tuning is necessary in the circuits shown in FIGS. 1A, 1B and 1C because they all use high dielectric constant materials to reduce the size of filter. This extensive post tuning is indicated by the large number of variable capacitances in the equivalent circuits of FIGS. 1A-C and by the large number of corresponding tuning areas which number as many as ten in Nakai et al.
In addition to the conventional circuit's susceptibility to layout variations and the subsequent need for extensive post tuning, there are two main drawbacks when these conventional circuits are practically applied. First, part of the filter components are exposed to the air which will affect the performance characteristics of the filter by energy coupling with peripheral circuits or components. Secondly, the conventional filter structure cannot be buried into the substrates and is difficult to integrate with other sub-modules to form a single, miniaturized, multifunctional module.
In summary, the main disadvantages of conventional filters are listed as follows.
a. Conventional filters use substrates with high dielectric constant to reduce the filter size which results in an extensive need for post tuning. PA1 b. Part of the conventional filter components are exposed to the air which will affect the filter characteristics because of energy coupling with peripheral circuits or components. PA1 c. The conventional filter structure cannot be buried into the substrate and which makes it difficult to integrate with other sub-modules to form a single, miniaturized, multifunction module. PA1 a. There is no need to use a substrate with high dielectric constant to reduce the inventive filter size, which will greatly reduce the amount of or even the need for post tuning. PA1 b. The inventive filter characteristics can be easily modified by adjusting the locations of loss poles to meet the required system specifications. Furthermore, adjusting the capacitance of the loss-pole shifting capacitor has little effect on bandwidth, central frequency, and insertion loss. PA1 c. The inventive filter is easy to design and fabricate for different bandwidth applications. PA1 d. The inventive filter has a construction that is suitable for burying into a substrate and, thus, is easy to integrate with other sub-modules to form a single, miniaturized, multifunction module.